Method for electrochemical oxidation

ABSTRACT

Disclosed is a method for the electrochemical oxidation of a semiconductor layer. In an electrochemical oxidation treatment for the production process of an electron source  10  (field-emission type electron source) as one of electronic devices, a control section  37  determines a voltage increment due to the resistance of an electrolytic solution B in advance, based on a detected voltage from a resistance detect section  35 . Then, the control section  37  controls a current source to supply a constant current so as to initiate an oxidation treatment for a semiconductor layer formed on an object  30 . The control section  37  corrects a detected voltage from a voltage detect section  36  by subtracting the voltage increment therefrom. When the corrected voltage reaches a given upper voltage value, the control section  37  is operable to discontinue the output of the current source  32  and terminate the oxidation treatment. The present invention allows electronic devices to be produced with reduced variation in the characteristics thereof.

TECHNICAL FIELD

The present invention relates to a method for the electrochemicaloxidation of semiconductors.

BACKGROUND ART

There has heretofore been known a wet anodization method as one oftechniques for providing porosity to a semiconductor or for forming anoxide film on the surface of a semiconductor. The techniques for formingan oxide film on the surface of a semiconductor also include anelectrochemical oxidation method utilizing an electrochemical reaction.In late years, there has been proposed a field emission-type electronsource prepared by a process using a wet anodization method and anelectrochemical oxidation method.

For example, as shown in FIG. 20, this kind of field emission-typeelectron source 10 (hereinafter referred to as “electron source 10” forbrevity) comprises an n-type silicon substrate 1 as a conductivesubstrate, and a strong-field drift layer 6 (hereinafter referred to as“drift layer 6” for brevity) which is composed of an oxidized porouspolycrystalline silicon layer and formed on the side of one of theprincipal surfaces of the n-type silicon substrate 1. Further, a surfaceelectrode 7 composed of a metal thin film (e.g. gold thin film) isformed on the drift layer 6, and an ohmic electrode 2 is formed on theback surface of the n-type silicon substrate 1. In this structure, then-type silicon substrate 1 and the ohmic electrode 2 serve as a lowerelectrode 12. While the electron source 10 illustrated in FIG. 20includes a non-doped polycrystalline silicon layer 3 interposed betweenthe n-type silicon substrate 1 and the drift layer 6, there has alsobeen proposed another electron source designed such that the drift layer6 is formed directly on the principal surface of the n-type siliconsubstrate 1.

In an operation of emitting electrons from the electron source 10illustrated in FIG. 20, a collector electrode 21 is disposed in opposedrelation to the surface electrode 7. Then, after a vacuum is formed inthe space between the surface electrode 7 and the collector electrode21, a DC voltage Vps is applied between the surface electrode 7 and thelower electrode 12 in such a manner that the surface electrode 7 has ahigher potential than that of the lower electrode 12. Simultaneously, aDC voltage Vc is applied between the collector electrode 21 and thesurface electrode 7 in such a manner that the collector electrode 21 hasa higher potential than that of the surface electrode 7. Each of the DCvoltages Vps, Vc can be appropriately arranged to allow electronsinjected from the lower electrode 12 into the drift layer 6 to beemitted through the surface electrode 7 after drifting in the driftlayer 6 (the one-dot chain lines in FIG. 20 indicate the flow of theelectrons e⁻ emitted through the surface electrode 7.). The surfaceelectrode 7 is made of a metal material having a small work function.

While the electron source 10 illustrated in FIG. 20 has the lowerelectrode 12 composed of the n-type silicon substrate 1 and the ohmicelectrode 2, there has also been proposed another electron source 10 asshown in FIG. 21, in which a lower electrode 12 made of a metal materialis formed on one of the principal surfaces of an insulative substrate11. The electron source 10 illustrated in FIG. 21 emits electrons in thesame process as that of the electron source 10 illustrated in FIG. 20.

Generally, in this kind of electron source 10, a current flowing betweenthe surface electrode 7 and the lower electrode 12 is referred to as“diode current Ips”, and a current flowing between the collectorelectrode 21 and the surface electrode 7 is referred to as “emissioncurrent (emitted electron current) Ie”. In the electron sources 10, anelectrode emission efficiency (=(Ie/Ips)×100[%]) becomes higher as theratio (Ie/Ips) of the emission current Ie to the diode current Ie isincreased. In this connection, the emission current Ie becomes higher asthe DC voltage Vps is increased. This electron source 10 exhibitselectron emission characteristics having a low dependence on the degreeof vacuum, and can stably emit electrons at a high electron emissionefficiency without occurrence of a so-called popping phenomenon.

If the electron source 10 illustrated in FIG. 21 is applied as anelectron source of a display, the display may be configured as shown inFIG. 22. The display illustrated in FIG. 22 comprises an electron source10, and a faceplate 30 which is composed of a flat-plate-shaped glasssubstrate and disposed in opposed relation to the electron source 10. Acollector electrode (hereinafter referred to as “anode electrode”) 21composed of a transparent conductive film (e.g. ITO film) is formed onthe surface of the faceplate 30 opposed to the electron source 10. Thesurface of the anode electrode 21 opposed to the electron source 10 isprovided with fluorescent materials formed in each of pixels, and blackstripes made of a black material and formed between the fluorescentmaterials. Each of the fluorescent materials applied on the surface ofthe anode electrode 21 opposed to the electron source 10 can generate avisible light in response to electron beams emitted from the electronsource 10. The electrons emitted from the electron source 10 areaccelerated by a voltage applied to the anode electrode 21, and thehighly energized electrons come into collision with the fluorescentmaterials. Three type of fluorescent materials having luminescent colorsof R (red), G (green) and B (blue) are used as the fluorescentmaterials. The faceplate 30 is spaced apart from the electron source 10by a rectangular frame (not shown), and an sealed space formed betweenthe faceplate 30 and the electron source 10 is kept in vacuum.

The electron source 10 illustrated in FIG. 22 comprises an insulativesubstrate 11 composed of a glass substrate, a plurality of lowerelectrodes 12 arranged in lines on the surface of the insulativesubstrate 11, a plurality of polycrystalline silicon layers 3 each ofwhich is formed on the corresponding lower electrode 12 in asuperimposed manner, a plurality of drift layers 6 each of which iscomposed of an oxidized porous polycrystalline silicon layer and formedon the corresponding polycrystalline silicon layer 3 in a superimposedmanner, a plurality of isolation layers 16 each of which is composed ofa polycrystalline silicon layer and embedded between the adjacent driftlayers 6, and a plurality of surface electrodes 7 which are formed onthe drift layers 6 and the isolation layers 16, and arranged in lines toextend in the crosswise direction of the lower electrodes 12 so as tocut across the drift layers 6 and the isolation layers 16.

In the electron source 10, the drift layers 6 are partly sandwichedbetween the corresponding lower electrodes 12 arranged on the surface ofthe insulative substrate 11 and the corresponding surface electrodes 7arranged in the crosswise direction of the lower electrodes 12, at theregions of the drift layers 6 corresponding to the intersecting pointsbetween the corresponding lower electrodes 12 and the correspondingsurface electrodes 7. Thus, a certain voltage can be applied betweenappropriately selected one of the plural pairs of the surface electrode7 and the lower electrode 12, to allow a strong electric field to act onthe region of the drift layer 6 corresponding to the intersecting pointbetween the selected surface electrode 7 and lower electrode 12 so as toemit electrons from the region. This configuration is equivalent to anelectron source in which a plurality of electron source elements 10 a,each of which comprises the lower electrode 12, the polycrystallinesilicon layer 3 on the lower electrode 12, the drift layer 6 on thepolycrystalline layer 3, and the surface electrode 7 on the drift layer6, are arranged, respectively, at the lattice points of a matrix(lattice) formed by a group of the surface electrodes 7 and a group ofthe lower electrodes 12 a. One of the pairs of the surface electrode 7and the lower electrode 12 to be applied with a certain voltage, can beselected to allow electrons to be emitted from desired one of theelectron source elements 10 a.

In a conventional production process for the electron source 10, thedrift layer 6 is formed through a film-forming step of forming anon-doped polycrystalline silicon layer on the side of one of thesurfaces of the lower electrode 12, an anodization step of anodizing thepolycrystailine silicon layer to form a porous polycrystalline siliconlayer containing polycrystal line silicon grains and nanometer-ordersilicon microcrystals, and an oxidation step of rapidly heating andoxidizing the porous polycrystalline silicon layer through a rapidheating method to form silicon oxide films on the surfaces of the grainsand the nanometer-order silicon microcrystals, respectively.

In the anodization step, a mixture prepared by mixing an aqueoussolution of hydrogen fluoride with ethanol at the ratio of about 1:1 isused as an electrolytic solution. In the oxidation step, a substrate isoxidized by increasing the substrate from room temperature up to 900° C.in a short period of time under a dry oxygen atmosphere, and maintainingthe substrate temperature at 900° C. for 1 hour, for example, using alamp annealing apparatus. Then, the substrate temperature is reduceddown to room temperature.

For example, a conventional anodization apparatus as shown in FIG. 24Ais used in the anodization step. This anodization apparatus comprises aprocessing both 31 containing a electrolytic solution A consisting amixture of ethanol and an aqueous solution of hydrogen fluoride, and acathode 33 composed of a grid-like platinum electrode and immersed intothe electrolytic solution A in the processing bath 31. An object 30having a polycrystalline silicon layer formed on the lower electrode 12is immersed into the electrolytic solution A, and the lower electrode 12is used as an anode. This anodization apparatus includes a currentsource 32 for supplying a current between the lower electrode 12 servingas an anode and the cathode 33 in such a manner that the anode has ahigher potential than that of the cathode. The anodization apparatusalso includes a light source (not shown) composed of a tungsten lamp forirradiating the principal surface of the object 30 (or the front surfaceof the polycrystalline silicon layer) with light.

A constant current is supplied between the anode and the cathode 33through an anodization method using the above anodization apparatus toprovide porosity from the surface of a target region E in thepolycrystalline silicon layer toward the depth direction thereof, so asto form a porous polycrystalline silicon layer containingpolycrystalline silicon grains and nanometer-order silicon microcrystalsin the target region.

As shown in FIG. 25, the electron source 10 illustrated in FIG. 22 maybe produced by arranging a plurality of lower electrodes 12 in lines onthe side of one of the principal surfaces of an insulative substrate 11,forming a polycrystalline silicon layer 3 on the side of the aboveprincipal surface of the insulative substrate 11, and anodizing therespective regions of the polycrystalline silicon layer 3 superimposedon the lower electrodes 12. In this process, a certain current issupplied to the lower electrode 12 through a current-feeding wiring 12 acontinuously extending in integral with the lower electrode 12.

In the oxidation step, the porous polycrystalline silicon layer israpidly heated and oxidized through the rapid heating method, asdescribed above. Differently from this method, a technique using anelectrochemical oxidation method of electrochemically oxidizing theporous polycrystalline silicon layer within an electrolytic solution(electrolyte solution) consisting of an aqueous solution of sulfuricacid, nitric acid or the like, in the oxidation step is proposed to forma silicon oxide film having an excellent film quality on all of thesurfaces of the silicon microcrystals and the grains. More specifically,in the drift layer 6, when the porous polycrystalline silicon layer isoxidized, a thin silicon oxide layer would be formed on each of thesurfaces of a number of silicon microcrystals and a number of grainscontained in the porous polycrystalline silicon layer. In view of thispoint, the proposed electrochemical oxidation method is intended to forma silicon oxide film having an excellent film quality on all of thesurfaces of the silicon microcrystals and the grains, byelectrochemically oxidizing the porous polycrystalline silicon layerwithin an electrolytic solution consisting, for example, of 1 mol/l ofaqueous solution of sulfuric acid, nitric acid or the like, in the stepof forming the drift layer 6.

The porous polycrystalline is electrochemically oxidized using anelectrochemical oxidation apparatus of FIGS. 23A and 23B, in which theelectrolytic solution A in the anodization apparatus of FIGS. 24A and24B is replaced with an electrolytic solution B consisting, for example,of an aqueous solution of sulfuric acid. As shown in FIG. 23B, a cathodeis set to have the same outside dimension as that of the target region Eof the polycrystalline silicon layer. With this electrochemicaloxidation apparatus, a certain current can be supplied from a currentsource 32 between the anode and the cathode 33 so as toelectrochemically oxidize the polycrystalline silicon layer in thetarget region E to form a silicon oxide films on each of the surfaces ofthe silicon microcrystals and the grains.

In the step of forming the porous polycrystalline silicon layer, theanodization treatment is completed after a certain current is suppliedbetween the anode and the cathode 33 just for a predetermined period oftime. By contrast, in the step of electrochemically oxidizing the porouspolycrystalline silicon layer, a certain current is supplied between theanode and the cathode 33, and the current supply is terminated at thetime when the voltage between the anode and the cathode 33 is increasedup to a predetermined value arranged depending on the characteristics(e.g. emission current or withstand voltage) of the electron source 10(see, for example, Japanese Patent Laid-Open Publication No.2001-155622)

As compared to the method of rapidly heating and oxidizing the porouspolycrystalline silicon layer to form the drift layer 6, theelectrochemical oxidation method allows the porous polycrystallinesilicon layer to be oxidized under a lowered process temperature. Thus,the restrictions on the material of the substrate can be reduced tofacilitate increase in the area and reduction in the cost of theelectron source 10.

On the other hand, the conventional electron source 10 produced usingthe aforementioned electrochemical oxidation method involves a problemof increased variation of the emission current Ie and/or withstandvoltage in the surface thereof, and resultingly deteriorated processyield. That is, an electronic device produced using the aforementionedelectrochemical oxidation method has a problem of wide variation intheir characteristics, such as emission current or withstand voltage.

The characteristics, such as emission current or withstand voltage,would be widely varied due to the following factors:

1) In the aforementioned electrochemical oxidation method, a voltageincrement due to the resistance of the electrolytic solution B isincluded in the voltage between the anode and the cathode. Thus, avoltage incitement due to the formation of the oxide films will bevaried according to the variation in the voltage increment caused by thevariation in the resistance of the electrolytic solution B.

2) As shown in FIG. 23B, the cathode 33 is set to have the same outsidedimension as that of the target region. Thus, a current flows throughthe electrolytic solution B by paths as shown by the arrows in FIG. 23A,and the peripheral portion of the target region E has a higher currentdensity than that in the remaining region thereof.

3) During electrochemical oxidation, air bubbles are formed on theprincipal surfaces of the porous polycrystalline silicon layer which isa semiconductor layer to suppress the reaction in the region having theair bubbles formed thereon.

The factor 1) leads to increased variation in the characteristics, suchas emission current or withstand voltage, mainly in each of processingbatches. The factor 2) or 3) leads to increased in-plane variation inthe characteristics, such as emission current or withstand voltage,mainly in a sample, and deteriorated process yield of electronicdevices.

DISCLOSURE OF INVENTION

In view the above problems, it is therefore an object of the presentinvention to provide an electrochemical oxidation method capable ofreducing the variation in characteristics, such as emission current orwithstand voltage, as compared to conventional methods.

In order to achieve the above object, the present invention provides amethod for the electrochemical oxidation of a semiconductor layer,wherein an electrode provided on the opposite side of the principalsurface of the semiconductor layer is used as an anode, and a current issupplied between the anode and a cathode while allowing thesemiconductor layer and the cathode to be in contact with anelectrolytic solution, to oxidize the semiconductor layer. In thiselectrochemical oxidation method, a current is first supplied betweenthe anode and the cathode to initiate the oxidation. Then, the oxidationis terminating under the condition that a corrected voltage value Vtdetermined by correcting a voltage V between the anode and the cathodein accordance with a voltage inclement V0 based on a pre-detectedresistance of the electrolytic solution is equal to a predeterminedupper voltage value V1.

According to the above electrochemical oxidation method, the variationin the increment of the voltage between the anode and the cathode in theperiod between the initiation and termination of the oxidation can bereduced irrespective of the resistance of the electrolytic solution.Thus, the variation of the voltage increment caused by the formation ofthe oxide films can be reduced to allow the characteristics of anelectronic device to have desirably suppressed variation.

In the electrochemical oxidation method, a current density in theprincipal surface of the semiconductor layer may be controlled in such amanner that the current density in the periphery of the oxidation targetregion of the semiconductor layer is restrained in increasing to begreater than the remaining oxidation target region. In this case, thein-plane variation of the current density in the oxidation target regioncan be reduced as compared to the conventional methods to allow thecharacteristics of an electronic device to have desirably suppressedin-plane variation.

Further, air bubbles formed on the principal surface of thesemiconductor layer during the supply of the current may be releasedfrom the principal surface while supplying the current. In this case,the oxidation target region can avoid the deterioration in a requiredreaction therein due to air bubbles to allow the characteristics of anelectronic device to have desirably suppressed in-plane variation.

BRIEF DESCRIPTION OF DRAWINGS

The present invention will be more sufficiently understood from thedetailed description and the accompanying drawings. In the accompanyingdrawings, common components or elements are defined by the samereference numerals or codes.

FIG. 1 is a schematic vertical sectional view of an electron source(field-emission-type electron source) concerning a first mode ofembodiment.

FIG. 2 is an explanatory diagram showing the operation of the electronsource in FIG. 1.

FIG. 3 is a schematic fragmentary enlarged vertical sectional viewshowing the electron source in FIG. 1.

FIGS. 4A to 4D are schematic vertical sectional views of the electronsource in FIG. 1 and intermediate products in major steps of aproduction process thereof, wherein a production method of the electronsource is explained in conjunction therewith.

FIG. 5 is a schematic front view of an electrochemical oxidationapparatus concerning the first mode of embodiment.

FIG. 6A is a graph showing the relationship between detected voltage Vand time in the electrochemical oxidation apparatus in FIG. 5.

FIG. 6B is a graph showing the relationship between corrected voltage Vtand time in the electron source in FIG. 1.

FIG. 7 is a schematic top plan view of an object concerning a secondmode of embodiment.

FIG. 8 is a schematic front view of an electrochemical oxidationapparatus for use in production steps of an electron source, concerninga third mode of embodiment.

FIG. 9 is a schematic front view of an electrochemical oxidationapparatus for use in production steps of an electron source, concerninga fourth mode of embodiment .

FIG. 10 is a graph showing the relationship between corrected voltage Vtand time in electrochemical oxidation apparatus concerning the fourthmode of embodiment.

FIGS. 11A to 11D are schematic vertical sectional views of an electronsource and intermediate products in major steps of a production processthereof, concerning a fifth mode of embodiment, wherein a productionmethod of the electron source is explained in conjunction therewith.

FIG. 12A is a schematic front view of the electrochemical oxidationapparatus concerning the fifth mode of embodiment.

FIG. 12B is a schematic fragmentary perspective view of theelectrochemical oxidation apparatus in FIG. 12A.

FIG. 13A is a schematic front view of an electrochemical oxidationapparatus concerning a sixth mode of embodiment.

FIG. 13B is a schematic fragmentary perspective view of theelectrochemical oxidation apparatus in FIG. 13A.

FIG. 13C is a schematic front view of another electrochemical oxidationapparatus concerning the sixth mode of embodiment.

FIG. 13D is a schematic fragmentary perspective view of theelectrochemical oxidation apparatus in FIG. 13C.

FIG. 14A is a schematic front view of an electrochemical oxidationapparatus concerning a seventh mode of embodiment.

FIG. 14B is a schematic fragmentary perspective view of theelectrochemical oxidation apparatus in FIG. 14A.

FIG. 15 is a fragmentary perspective view of a display using an electronsource concerning an eighth mode of embodiment.

FIG. 16 is a perspective view of an intermediate product in major stepsof a production process of a display using the electron source in FIG.15.

FIGS. 17A to 17D are schematic vertical sectional views of an electronsource and intermediate products in major steps of a production processthereof, concerning a nine mode of embodiment, wherein a productionmethod of the electron source is explained in conjunction therewith.

FIG. 18 is a schematic front view of an electrochemical oxidationapparatus concerning the nine mode of embodiment.

FIG. 19 is a schematic front view of an electrochemical oxidationapparatus concerning a tenth mode of embodiment.

FIG. 20 is an explanatory diagram showing the operation of aconventional electron source.

FIG. 21 is an explanatory diagram showing the operation of anotherconventional electron source.

FIG. 22 is a schematic perspective view of a display using the electronsource in FIG. 21.

FIG. 23A is a schematic front view of a conventional electrochemicaloxidation apparatus.

FIG. 23B is a schematic fragmentary perspective view of theelectrochemical oxidation apparatus in FIG. 23A.

FIG. 24A is a schematic front view of an anodization apparatus.

FIG. 24B is a schematic perspective view of the anodization apparatus inFIG. 24A.

FIG. 25 is a perspective view of an intermediate product in a major stepof a production process of a display using a conventional electronsource.

BEST MODE FOR CARRYING OUT THE INVENTION

This application is based upon and claims the benefit of priority fromthe prior Japanese Patent Application No. 2002-138993, the entirecontents of which are incorporated herein by reference.

Several modes of embodiment of the present invention will now bespecifically described. Members common in each mode of embodiment ormembers having substantially the same structure and function are definedby the same reference numerals, and duplicate descriptions will beomitted.

[First Mode of Embodiment]

A first mode of embodiment will be described by taking an electronsource (field-emission-type electron source) for example, as one ofelectronic devices produced using an electrochemical oxidation method ofthe present invention.

As shown in FIG. 1A, the electron source 10 concerning the first mode ofembodiment comprises an electron source element 10 a formed on the sideof one of the principal surfaces of a substrate 11 composed of aninsulative substrate (e.g. a glass substrate having an insulationperformance or a ceramic substrate having an insulation performance).The electron source element 10 a includes a lower electrode 12 formed onthe side of the above principal surface of the substrate 11, a non-dopedpolycrystalline silicon layer 3 formed on the lower electrode 12, adrift layer 6 (strong-field drift layer) formed on the polycrystallinesilicon layer 3, and a surface electrode 7 formed on the drift layer 6.That is, in the electron source element 10 a, the surface electrode 7 isdispose in opposed relation to the lower electrode 12, and the driftlayer 6 is interposed between the surface electrode 7 and the lowerelectrode 12. While an insulative substrate is used as the substrate 11in the first mode of embodiment, a semiconductor substrate such as asilicon substrate may be used as the substrate. In this case, the lowerelectrode may comprise the semiconductor substrate, and a conductivelayer (e.g. ohmic electrode) laminated on the back surface of thesemiconductor layer. Further, while the polycrystalline silicon layer 3is interposed between the drift layer 6 and the lower electrode 12, thedrift layer 6 may be formed directly on the lower electrode 12.

The lower electrode 12 is formed of a single-layer thin film made ofmetal material (e.g. Mo, Cr, W, Ti, Ta, Ni, Al, Cu, Au or Pt; alloythereof; or intermetallic compound such as silicide). The lowerelectrode 12 may also be formed of a multilayer thin film made of theabove metal materials. The thickness of the lower electrode 12 is set atabout 300 nm.

While the surface electrode 7 is made of a material having a smallfunction, for example gold, the material of the surface electrode 7 isnot limited to gold. The surface electrode is not limited to asingle-layer structure, but may be formed as a multilayer structure. Thesurface electrode 7 may be set at any thickness allowing electrons fromthe drift layer 6 to tunnel therethrough, for example, in the range ofabout 10 to 15 nm.

As shown in FIG. 2, in an operation of emitting electrons from theelectron source 10, a collector electrode 21 is disposed in opposedrelation to the surface electrode 7. Then, after a vacuum is formed inthe space between the surface electrode 7 and the collector electrode21, a DC voltage Vps is applied between the surface electrode 7 and thelower electrode 12 in such a manner that the surface electrode 7 has ahigher potential than that of the lower electrode 12. Simultaneously, aDC voltage Vc is applied between the collector electrode 21 and thesurface electrode 7 in such a manner that the collector electrode 21 hasa higher potential than that of the surface electrode 7. Each of the DCvoltages Vps, Vc can be appropriately arranged to allow electronsinjected from the lower electrode 12 into the drift layer 6 to beemitted through the surface electrode 7 after drifting in the driftlayer 6 (the one-dot chain lines in FIG. 2 indicate the flow of theelectrons e⁻ emitted through the surface electrode 7.). Since electronsreaching the surface of the drift layer 6 would be hot electrons, theycan readily tunnel through the surface electrode 7 and burst out into avacuum space. In this electron sources 10, an electrode emissionefficiency (=(Ie/Ips)×100[%]) becomes higher as the ratio (Ie/Ips) of anemission current Ie to a diode current Ie is increased.

As shown in FIG. 3, the drift layer 6 is formed through ananocrystallization process using an anodization method and an oxidationprocess using an electrochemical oxidation method, as described later.It is believed that the drift layer 6 comprise at least a plurality ofcolumnar polycrystalline silicon grains (semiconductor crystals) 51arrayed in lines on the side of the above principal surface of the lowerelectrode 12, a plurality of thin silicon oxide films 52 each formedover the surface of the corresponding grain 51, a number ofnanometer-order silicon microcrystals (semiconductor microcrystal) 63residing between the adjacent grains 51, and a number of silicon oxidefilms (insulative films) 64 each formed over the surface of thecorresponding silicon microcrystal 63. Each of the silicon oxide filmshas a film thickness less than the grain size of the correspondingsilicon microcrystal 63. Each of the grains 51 extends in the thicknessdirection of the lower electrode 12.

In the electron source 10, the electron emission would be caused basedon the following mode. A DC voltage Vps is applied between the surfaceelectrode 7 and the lower electrode 12 in such a manner that the surfaceelectrode 7 has a higher potential than that of the lower electrode 112,and simultaneously a DC voltage Vc is applied between the collectorelectrode 21 and the surface electrode 7 in such a manner that thecollector electrode 21 has a higher potential than that of the surfaceelectrode 7. When the DC voltage Vps reaches a given value (criticalvalue), electrons e⁻ are injected from the lower electrode 12 into thedrift layer 6. At the same time, the electric field applied to the driftlayer 6 mostly acts to the silicon oxide films 64. Thus, the electronse⁻ injected into the drift layer 6 are accelerated by the strongelectric field acting on the silicon oxide films 64. In the drift layer6, the electrons then drift toward the surface electrode or in thedirection of the arrows in FIG. 3, through the region between theadjacent grains 51. After tunneling through the surface electrode 7, theelectrons are emitted to the vacuum space. In this manner, the electronsinjected from the lower electrode 12 into the drift layer 6 areaccelerated by the electric field acting on the silicon oxide films 64,and then emitted through the surface electrode 7 after driftingapproximately without scattering due to the silicon microcrystals 63.Further, heat generated at the drift layer 6 is released through thegrains 51. Thus, during the electron emission, the electrons can bestably emitted without occurrence of the popping phenomenon.

In a production process of this electron source 10, a lower electrode 12composed of a metal film (e.g. tungsten film) having a given filmthickness (e.g. about 300 nm) is first formed on one of the principalsurfaces of a substrate 11 composed of a glass substrate having aninsulation performance. Then, a non-doped polycrystalline silicon layer3 having a given film thickness (e.g. 1.5 μm) is formed on the entireprincipal surface of the substrate 11, for example, through a plasma CVDmethod, to obtain a structure (intermediate product) as shown in FIG.4A. The film-forming method for the polycrystalline silicon layer 3 isnot limited to the plasma CVD method, but any other suitablefilm-forming method, such as a LPCVD method, a catalytic-CVD method, asputtering method or a CGS (Continuous Grain Silicon) method, may beused.

After the formation of the non-doped polycrystalline silicon layer 3,the nanocrystallization process is performed to form a compositenanocrystal layer 4 including a number of polycrystalline silicon grains51 (see FIG. 3) and a number of silicon microcrystals 63 (see FIG. 3)which are mixed together. In this manner, a structure as shown in FIG.4B is obtained. The nanocrystallization process is performed using ananodization apparatus as shown in FIGS. 24A and 24B. The anodizationapparatus comprises a processing bath containing an electrolyticsolution A formed as a mixture of an aqueous solution of 55 wt %hydrogen fluoride and ethanol which are mixed at the ratio of about 1:1.A given current (e.g. a current having a current density of 12 mA/cm²)is supplied between a platinum electrode serving as a cathode and thelower electrode 12 serving as an anode, for a given time (e.g., 10seconds) while irradiating the polycrystalline silicon layer 3 withlight to form the composite nanocrystal layer 4. This compositenanocrystal layer 4 includes the polycrystalline silicon grains 51 andthe silicon microcrystals 63. In the first mode of embodiment, thecomposite nanocrystal layer 4 serves as a semiconductor layer.

After the completion of the nanocrystallization process, the oxidationprocess is performed. Through this process, the drift layer 6 composedof a composite nanocrystal layer with a structure as shown in FIG. 3 isformed to obtain a structure as shown in FIG. 4C.

The oxidation process is performed using an electrochemical oxidationapparatus as shown in FIG. 5. Specifically, an object 30 formed with thecomposite nanocrystal layer 4 is immersed into an electrolytic solution(e.g. a solution comprising an organic solvent of ethylene glycol, and asolute of 0.04 mol/l potassium nitrate dissolved in the organic solvent)B contained in a processing bath 31. Then, a cathode 33 composed of agrid-like platinum electrode is disposed in opposed relation to thecomposite nanocrystal layer 4 within the electrolytic solution B. Then,a constant current (e.g. a current having a current density of 0.1mA/cm²) is supplied from a current source 32 between the lower electrode12 used as an anode and a cathode 33. In this manner, the oxidationprocess for electrochemically oxidizing the composite nanocrystal layer4 is performed to form a drift layer 6 including grains 51, siliconmicrocrystals 63 and silicon oxide films 52, 64.

This electrochemical oxidation apparatus includes: a resistance detectsection 35 for detecting the resistance of the electrolytic solution Bcontained in a processing bath 31, by use of a pair ofresistance-measuring electrodes 34 a, 34 b to be immersed into theelectrolytic solution B; a voltage detect section 36 for detecting thepotential difference between the anode and the cathode 33; and a controlsection 37 for controlling the output of the current source 32 inaccordance with a detected voltage from the voltage detect section 36and a detected resistance value from the resistance detect section 35.The control section 37 is operable to determine a voltage increment V0(see FIG. 6A) to be caused by the resistance of the electrolyticsolution B in accordance with a detected resistance value which isdetected by the resistance detect section 35 in advance, and then tocontrol the current source 32 such that a constant current is suppliedfrom the current source 32, so as to initiate an oxidation treatment.The control section 37 is also operable to correct the detected voltageV from the voltage detect section 36 by subtracting the voltageincrements V0 from the detected voltage V, and the to discontinue theoutput of the current source 32 at the time when the corrected voltageVt reaches a predetermined upper voltage value V1 (see FIG. 6B) so as toterminate the oxidation treatment. In the composite nanocrystal layer 4formed through the nanocrystallization process in the first mode ofembodiment, any remaining region other than the grains 51 and thesilicon microcrystals 63 is formed as an amorphous silicon region madeof amorphous silicon. In the drift layer 6, the remaining region otherthan the grains 51, the silicon microcrystals 63, and the silicon oxidefilms 52, 64 is formed as an amorphous region 65 made of amorphoussilicon or partially oxidized amorphous silicon. However, depending onthe conditions of the nanocrystallization process, the amorphous region65 is formed as pores. In this case, such a composite nanocrystal layer4 may be considered as a porous polycrystalline silicon layer as inconventional electron sources.

After the formation of the drift layer 6, a surface electrode 7 composedof a gold thin film is formed on the drift layer 6, for example, througha vapor deposition method.

In the conventional electrochemical oxidation method for forming thedrift layer 6, the oxidation treatment is terminated at the time whenthe voltage between the anode and the cathode 33 reaches a given voltage(V0+V1), as shown in FIG. 6A. In this case, the given voltage includes avoltage increment (V1) caused by the formation of the oxide films(silicon oxide films 52, 64), and another voltage increment V0 due tothe resistance of the electrolytic solution B. The voltage increment V0is varied according to the variation in the specific resistance of theelectrolytic solution B depending on the conditions of the electrolyticsolution B, such as production conditions, continuous-use conditions orstorage conditions; the shape of a cathode electrode; and the surfacecondition of a sample. Thus, if the given voltage (V0+V1) is a constantvalue, the voltage increment caused by the formation of the siliconoxide films 52, 64 are varied, and consequently the emission currentand/or withstand voltage of the electron source 10 are varied, resultingin undesirably deteriorate process yield.

By contrast, according to the production method of the first mode ofembodiment, in the electrochemical oxidation method forelectrochemically oxidizing the composite nanocrystal layer 4 as acrystal layer to form the drift layer 6, a certain current is suppliedbetween the anode and the cathode 33 to initiate the oxidation. Then,the oxidation is terminated at the time when the corrected voltage Vtdetermined by correcting the voltage between the anode and the cathode33 in accordance with the voltage increment V0 based on the pre-detectedresistance of the electrolytic solution B reaches the upper voltagevalue V1. Thus, the variation in the increment of the voltage betweenthe anode an the cathode 33 in the period between the initiation andtermination of the oxidation can be reduced irrespective of thevariation in the resistance of the electrolytic solution B depending onthe conditions of the electrolytic solution B, such as productionconditions, continuous-use conditions or storage conditions. That is,the variation in the voltage increment caused by the formation of theoxide films (silicon oxide film 52, 64) can be reduced to providereduced variation in the characteristics, such as emission current orwithstand voltage, of the electron source 10.

In the electrochemical oxidation method according to the first mode ofembodiment, before the current is supplied between the anode and thecathode 33, the resistance of the electrolytic solution is detectedusing the resistance-measuring electrodes 34 a, 34 b, and the voltageincrement V0 is determined based on the detected resistance value. Inthis operation of detecting the resistance of the electrolytic solution,the voltage increment V0 can be obtained without supplying any currentbetween the anode and the cathode 33, to prevent the compositenanocrystal layer 4 as a semiconductor layer (crystal layer) from beingoxidized during this operation. Preferably, the distance between theresistance-measuring electrodes 34 a, 34 b is set to be equal to thedistance between the object 30 and the cathode 33. Further, in the firstmode of embodiment, a parameter corresponding to the distance betweenthe object 30 and the cathode 33, or the shape or dimension of thecathode 33 is entered into the control section 37. Based on thisparameter and the detected resistance value from the resistance detectsection 35, the control section 35 determines the specific resistance ofthe electrolytic solution B. Then, the control section 35 determines thevoltage increment V0 in accordance with the obtained specificresistance.

The electrolytic solution B for use in the electrochemical oxidationmethod comprises an organic solvent, and an electrolyte dissolved in theorganic solvent. Thus, as compared to a conventional process ofelectrostatically oxidizing a semiconductor layer in the electrolyticsolution consisting of an aqueous solution of sulfuric acid or nitricacid to form a silicon oxide film, the electrolytic solution B canprevent water from being incorporated in the oxide films to providehighly densified silicon oxide films 52, 64. Thus, the obtained siliconoxide films 52, 64 can have higher withstand voltage performance. Asdescribed above, if an organic solvent is used in an electrolyticsolvent B, the resistance of the electrolytic solution B will often beextremely increased as compared to an electrolytic solution using wateras a solvent. This tendency stands out when a nonpolar organic solventis use. Thus, the electrochemical oxidation method according to thefirst mode of embodiment is particularly effective to anorganic-solvent-based electrolytic solution B having a high resistancewhich leads to larger voltage increment V0.

When the electron source 10 concerning the first mode of embodiment isused as an electron source of a display, a number of electron sourceelements 10 a may be formed on the side of the above principal surfaceof the substrate 11 in a matrix arrangement by appropriately patterningthe lower electrode 12, the surface electrode 7, drift layer 6 andothers. While the first mode of embodiment has been described by takingthe production process of the electron source 10 for example, it isunderstood that the electrochemical oxidation method of the presentinvention is not limited thereto, but may be used in the productionprocess of various semiconductor devices.

[Second Mode of Embodiment]

A second mode of embodiment of the present invention will be describedbelow. As mentioned above, in the production process of the electronsource 10 concerning the first mode of embodiment, the resistance of theelectrolytic solution B is measured by the resistance-measuringelectrodes 34 a, 34 b of the electrochemical oxidation apparatus beforethe oxidation treatment. Then, based on the pre-detected resistance, thedetected voltage from the voltage detect section 36 is corrected inconsideration of the distance between the object 30 and the cathode 33,and the shape of the cathode 33. However, after the detection, theresistance of the electrolytic solution B can be changed in some casesdepending on the surface condition of the object 30.

In view of this possibility, as shown in FIG. 7, in the second mode ofembodiment, a resistance-measuring region 30 b is provided in theprincipal surface of a semiconductor layer (composite nanocrystal layer4) formed the an object 30, in addition to or separately from a desiredoxidation target region 30 a. Before a certain current is suppliedbetween the anode and the cathode 33, the resistance of the electrolyticsolution B is detected using the resistance-measuring region 30 b, andthe voltage increment V0 is determined based on a detected resistancevalue. The detected value from the voltage detect section 36 iscorrected in accordance with the determined voltage increment V0. Inthis point, the second mode of embodiment is different from the firstmode of embodiment.

According to the second mode of embodiment, the resistance of theelectrolytic solution B can be detected in the form of including thefactor of the surface condition of the composite nanocrystal layer 4 asa semiconductor while preventing the composite nanocrystal layer 4 frombeing oxidized during the detection of the resistance of theelectrolytic solution B. Thus, the difference between the detectedresistance value and the actual resistance value of the electrolyticsolution at the initiation of current supply can be reduced to providemore reduced variation in the characteristics of the electron source 10.The structure and operation of the electron source 10 are the same asthose in the first mode of embodiment, and their drawing and descriptionare omitted.

[Third Mode of Embodiment]

A third mode of embodiment of the present invention will be describedbelow. As mentioned above, in the electrochemical oxidation methodaccording to the second mode of embodiment, it is required to providethe resistance-measuring region 30 b to the object 30 in addition to theoxidation target region 30 a. However, depending on the pattern shape ofthe oxidation target region 30 a and other factor, anyresistance-measuring region 30 b cannot be provided in some cases, orleader lines cannot be adequately arranged in the resistance-measuringregion 30 b in some cases.

In view of this possibility, an electrochemical oxidation apparatus asshown in FIG. 8 is used in the third mode of embodiment. In theoxidation treatment of an object 30, before a certain-current issupplied between an anode (lower electrode 12) and a cathode 33, theresistance of an electrolytic solution B is detected using aresistance-monitoring specimen (not shown) which is formed in a shapeidentical to that of the object 30 formed with a semiconductor layer(composite nanocrystal layer 4) to be electrochemically oxidized. Thevoltage increment V0 is determined based on the detected resistancevalue. In this point, the third mode of embodiment is different from thesecond mode of embodiment.

According to the third mode of embodiment, before a certain current issupplied between the anode and the cathode 33, the resistance of theelectrolytic solution B is detected using the resistance-monitoringspecimen, and the voltage increment V0 is determined based on thedetected resistance value. Thus, any oxidation of the compositenanocrystal layer 4 can be avoided during the detection of theresistance of the electrolytic solution B. Further, the differencebetween the detected resistance value and the actual resistance value ofthe electrolytic solution at the initiation of current supply can bereduced to provide more reduced variation in the characteristics of theelectron source 10. The structure and operation of the electron source10 are the same as those in the first mode of embodiment, and theirdrawing and description are omitted.

[Fourth Mode of Embodiment]

A fourth mode of embodiment of the present invention will be describedbelow. As mentioned above, in the electrochemical oxidation methodaccording to the first to third modes of embodiment, the oxidationtreatment is terminated at the time when the voltage between the anode(lower electrode 12) and the cathode 33 reaches the upper voltage valueV1. In this case, as compared to the silicon oxide films 52, 64 formedat a position close to the lower electrode 12, the silicon oxide films52, 64 formed at a position far from to the lower electrode 12 cannothave adequate withstand voltage performance due to its excessively thinfilm thickness or insufficient density, in some cases.

In view of this possibility, an electrochemical oxidation apparatus asshown in FIG. 9 is used in the fourth mode of embodiment. As shown inFIG. 10, after the corrected voltage Vt between the anode and thecathode 33 reaches the upper voltage V1, the current I flowing betweenthe anode and the cathode 33 is reduced while maintaining the correctedvoltage Vt at the upper voltage value. Then, the oxidation is terminatedat the time when the current I is reduced down to a given value I1.

As shown in FIG. 9, the electrochemical oxidation apparatus concerningthe fourth mode of embodiment includes a current detect section 39 fordetecting the current flowing between the anode and the cathode 33,through a current censor 38. This electrochemical oxidation apparatusfurther includes a current source 32, a voltage source 40, and aselector switch 41 for selectively switching between the current source32 and the voltage source 40. In advance of supplying a current, acontrol section 37 corrects the upper voltage value V1 in accordancewith the voltage increment V0 based on the resistance of an electrolyticsolution B. During the reduction of the current I, the control sectioncontinuously corrects the corrected voltage Vt in accordance with thevoltage increment V0 which is a product of the current flowing throughthe electrolytic solution B and the resistance of the electrolyticsolution B. The above technical concept of the forth mode of embodimentmay be applied to the electrochemical oxidation apparatus concerning thesecond and third modes of embodiment.

According to the fourth mode of embodiment, the variation in the voltageincrement in the period between the initiation of current supply and thetime when the voltage between the anode and the cathode reaches theupper voltage value can be reduced. Further, in the period between thetime when the voltage between the anode and the cathode reaches theupper voltage value and the time when the current is reduced down to thegiven value I1, the corrected voltage Vt is corrected in response to thechange of the current flowing the electrolytic solution. Thus in theabove period, the variation in the voltage increment caused by theformation of the oxide films can be suppressed to provide reducedvariation in the characteristics of the electron source 10. Furthermore,since the oxidation is terminated only after the current I is reduceddown to the given value, the oxide films can be densified to have higherwithstand voltage performance.

[Fifth Mode of Embodiment]

A fifth mode of embodiment of the present invention will be describedbelow. An electrochemical oxidation method according to the fifth modeof embodiment will be described by taking the same electron source asthat in the first mode of embodiment for example, as one of electronicdevices to be formed through the electrochemical oxidation method. Thatis, the structure, function, advantages and operation of electronemission of the electron source 10 concerning the fifth mode ofembodiment is the same as those of the electron source 10 in the firstmode of embodiment (see FIGS. 1 to 3).

With reference to FIGS. 11A to 11D, a production process of the electronsource 10 concerning the fifth mode of embodiment will be describedbelow.

In this production process, a lower electrode 12 composed of a metalfilm is first formed on one of the principal surfaces of an insulativesubstrate 11, and then a non-doped polycrystalline silicon layer 3 isformed on the entire principal surface of the insulative substrate 11 toobtain a structure as shown in FIG. 11A, in the same way as that in thefirst mode of embodiment.

After the formation of the polycrystalline silicon layer 3, ananocrystallization process (anodization step) is performed to form acomposite nanocrystal layer 4 including a number of polycrystallinesilicon grains 51 (see FIG. 3) and a number of silicon microcrystals 63(see FIG. 3) which are mixed together. In this manner a structure asshown in FIG. 11B is obtained. The nanocrystallization process is thesame as that in the first mode of embodiment.

After the completion of the nanocrystallization process, an oxidationprocess is performed to electrochemically oxidize the compositenanocrystal layer 4. Through this process, a drift layer 6 with astructure as shown in FIG. 3 is formed to obtain a structure as shown inFIG. 11C. The oxidation process (oxidation step) is performed using theaforementioned electrochemical oxidation apparatus in FIG. 12A toelectrochemically oxidize the composite nanocrystal layer 4 as asemiconductor layer (crystal layer). This electrochemical oxidationapparatus has fundamentally the same structure as that of theconventional electrochemical oxidation apparatus in FIG. 23, except thatthe shape of a cathode 33 is adjusted to control a current density inthe principal surface of the polycrystalline silicon layer 3, so thatthe current density in the periphery of a target region E is restrainedin increasing to be greater than that in the remaining target region E.

Specifically, as shown in FIGS. 12A and 12B, a grid-like cathode 33 isformed to have an outer dimension less than that of the target region E(oxidation target region), so that the current density in the peripheryof the polycrystalline silicon layer 3 is restrained in increasing to begreater than that in the remaining region other than the target regionE. In other words, the shape of the cathode 33 is determined such thatthe specific surface area per unit area in the periphery of the cathodeis less than that in the remaining region of the cathode 33, to allowthe entire surface of the target region E to have an even currentdensity. In this point, the electrochemical oxidation method accordingto the fifth mode of embodiment is different from the conventionalelectrochemical oxidation method.

In the oxidation process, a solution comprising an organic solvent, e.g.ethylene glycol, and a solute of 0.04 mol/l potassium nitrate dissolvedin the organic solvent is used as a specific electrolytic solution B tobe contained in a processing bath 31. An object 30 formed with thecomposite nanocrystal layer 4 is immersed into the electrolytic solutionB. Then, the cathode 33 is disposed in opposed relation to the compositenanocrystal layer 4 within the electrolytic solution B. Then, a constantcurrent (e.g. a current having a current density of 0.1 mA/cm²) issupplied from the current source between an anode (lower electrode 12)and the cathode 33 to perform the oxidation process forelectrochemically oxidizing the composite nanocrystal layer 4. Throughthis process, a drift layer 6 including grains 51, silicon microcrystals63 and silicon oxide films 52, 64 is formed.

In the composite nanocrystal layer 4 formed through thenanocrystallization process in the fifth mode of embodiment, anyremaining region other than the grains 51 and the silicon microcrystals63 is formed as an amorphous silicon region made of amorphous silicon.In the drift layer 6, the remaining region other than the grains 51, thesilicon microcrystals 63 and the silicon oxide films 52, 64 is formed asan amorphous region 65 made of amorphous silicon or partially oxidizedamorphous silicon. However, depending on the conditions of thenanocrystallization process (anodization step), the amorphous region 65is formed as pores. In this case, such a composite nanocrystal layer 4may be considered as a porous polycrystalline silicon layer as inconventional electron sources.

After the formation of the drift layer 6, a surface electrode 7 composedof a gold thin film is formed on the drift layer 6, for example, througha vapor deposition method. In this way, an electron source 10 with astructure as shown in FIG. 11D is obtained.

According to the production process of the electron source 10 concerningthe fifth mode of embodiment, the current density in theelectrochemical-oxidation target region E as the principal surface of asemiconductor layer can be controlled such that the current density inthe periphery of the target region E is restrained in increasing to begreater than that in the remaining target region E. Thus, as compared tothe conventional method, the in-plane variation in the current densityof the target region E can be reduced. That is, as compared to theconventional method, the in-plane variation in the emission current Ieof the electron source 10 can be reduced, or the in-plane variation inthe characteristics of electron devices can be reduced. In addition, thecurrent density in the target region E as the principal surface of asemiconductor layer can be controlled only by adjusting the shape of thecathode 33. Thus, as compared to the conventional method, the in-plainvariation in the current density of the target region E can be reducedonly by adjusting the shape of the cathode 33, to provide reducedin-plane variation in the emission current Ie of the electron source 10at a low cost.

The above technical concept can be applied to a nanocrystallizationprocess using an anodization method.

[Sixth Mode of Embodiment]

A sixth mode of embodiment of the present invention will be describedbelow. As mentioned above, in the electrochemical oxidation methodaccording to the fifth mode of embodiment, the cathode 33 is formed tohave an outer dimension less than that of the target region E so as toprovide enhanced in-plane evenness in the current density of the targetregion E. However, depending on the distance between the target region Eand the cathode 33 or the specific resistance of the electrolyticsolution, this method cannot sufficiently equalize the current densityof the target region E in some cases, because the parallel linesconstituting the cathode 33 have the same pitch.

In view of this possibility, in the sixth mode of embodiment, anelectrochemical oxidation apparatus as shown in FIG. 13 is used toelectrochemically oxidize the target region E of an object 30. Theconstruction and operation of an electron source 10 concerning the sixthmode of embodiment is the same as those of the electron source 10 in thefirst mode of embodiment, and their drawing and description will beomitted.

The electrochemical oxidation apparatus concerning the six mode ofembodiment has fundamentally the same structure as that of theelectrochemical oxidation apparatus in the fifth mode of embodimentexcept for the shape of the cathode 33. As shown in FIGS. 13A and 13B,the outer dimension of the cathode 33 is approximately the same as thatof the target region E as in the conventional apparatus. The adjacentparallel linens of the cathode 33 are arranged to have a larger pitch inthe periphery of the cathode 33 than that in the central zone of thecathode 33, so as to provide enhanced evenness in the current density ofthe target region E. In other words, in the sixth mode of embodiment,the pitch of the adjacent parallel lines of the grid-like cathode 33 ischanged in such a manner that the current density in the periphery ofthe target region E is restrained in increasing to be greater than thatin the remaining target region E. More specifically, the cathode 33 isadjusted to have a shape allowing the specific surface area per unitarea in the periphery of the cathode 33 to be less than that of theremaining target region 33, so as to control the current density of thetarget region E as the principal surface of a semiconductor layer.

In the sixth mode of embodiment, an electrochemical oxidation apparatusas shown in FIGS. 13C and 13D may also be used. The cathode 33 in thisapparatus is formed to have a shape allowing the distance between thecathode 33 and the target region E to be increased in the periphery ofthe cathode 33. Thus, the resistance of the electrolytic solution B inthe periphery of the cathode 33 becomes higher than that in theremaining region of the cathode 33, so as to provide enhanced evennessin the current density of the target region E. That is, the distancebetween the cathode and the target region E (treatment region) isadjusted to change the resistance due to the electrolytic solution B inbetween the central zone and peripheral zone of the target region E soas to control the current density of the target region E.

In this way, as with the fifth mode of embodiment, the six mode ofembodiment can control the current density of the principal surface ofthe semiconductor layer (polycrystalline silicon layer 3 and compositenanocrystal layer 4) in such a manner that the current density in theperiphery of the electrochemical-oxidation target region is restrictedin increasing to be greater than that in the remaining target region E.Thus, as compared to the conventional method, the in-plane variation inthe current density of the target region E can be reduced to providereduced in-plane variation in the emission current Ie of the electronsource 10. In addition, the current density in the semiconductor layercan be controlled only by adjusting the shape of the cathode 33. Thus,as compared to the conventional method, the in-plain variation in thecurrent density of the target region E can be reduced only by adjustingthe shape of the cathode 33, to provide reduced in-plane variation inthe emission current Ie of the electron source 10 at a low cost.

The above technical concept can be applied to a nanocrystallizationprocess using an anodization method.

[Seventh Mode of Embodiment]

A seventh mode of embodiment of the present invention will be describedbelow. As mentioned above, in the fifth and sixth modes of embodiment,the shape of the cathode 33 is adjusted to equalize the current densityof the target region E. In this case, the shape of the cathode 33 has tobe designed in conformity with the shape of the target region E.

In view of this point, in the seventh mode of embodiment, anelectrochemical oxidation apparatus as shown in FIG. 14 is used toelectrochemically oxidize the target region E of a semiconductorcomposed of a polycrystalline silicon layer 3 in an object 30. Theconstruction and operation of an electron source concerning the seventhmode of embodiment is the same as those of the electron source in thefifth mode of embodiment, and their drawing and description will beomitted.

The electrochemical oxidation apparatus concerning the seventh mode ofembodiment is fundamentally the same as that in the fifth mode ofembodiment. As shown in FIGS. 14A and 14B, in the seventh mode ofembodiment, a dummy region D capable of reducing a current density isprovided in the periphery of a target region of a semiconductor layer,so as to control the current density of the principal surface of thesemiconductor. Thus, as compared to the conventional method, thein-plane variation in the current density of the target region can bereduced without modifying the shape of the cathode 33, to providereduced in-plane variance in the emission current Ie of the electronsource 10 at a low cost. The dummy region D is made of the same materialas that of the target region, and may be formed in conjunction with thetarget region E.

[Eighth Mode of Embodiment]

An eighth mode of embodiment of the present invention will be describedbelow. An electron source 10 concerning the eighth mode of embodimenthas approximately the same structure as that of the conventionalelectron source 10 in FIG. 22. Specifically, as shown in FIG. 15, theelectron source 10 comprises a plurality of lower electrodes 12 arrangedin lines on one of the principal surfaces of an insulative substrate 11,a plurality of polycrystalline silicon layers 3 each of which is formedon the corresponding lower electrode 12 in a superimposed manner, aplurality of drift layers 6 each of which is formed on the correspondingpolycrystalline silicon layer 3 in a superimposed manner, a plurality ofisolation layers 16 each of which is composed of a polycrystallinesilicon layer and embedded between the adjacent drift layers 6, and aplurality of surface electrodes 7 which are formed on the drift layers 6and the isolation layers 16, and arranged in lines to extend in thecrosswise direction of the lower electrodes 12 so as to cut across thedrift layers 6 and the isolation layers 16. As with the fifth mode ofembodiment, each of the drift layer 6 is composed of a compositenanocrystal layer.

As with the conventional electron source 10, in the electron source 10concerning the eighth mode of embodiment, the drift layers 6 are partlysandwiched between the corresponding lower electrodes 12 arranged on theabove principal surface of the insulative substrate 11 and thecorresponding surface electrodes 7 arranged in the crosswise directionof the lower electrodes 12, at the regions of the drift layers 6corresponding to the intersecting points between the corresponding lowerelectrodes 12 and the corresponding surface electrodes 7. Thus, acertain voltage can be applied between appropriately selected one of theplural pairs of the surface electrode 7 and the lower electrode 12, toallow a strong electric field to act on the region of the drift layer 6corresponding to the intersecting point between the selected surfaceelectrode 7 and lower electrode 12 so as to emit electrons from theregion. This configuration is equivalent to an electron source in whicha plurality of electron source elements 10 a, each of which comprisesthe lower electrode 12, the polycrystalline silicon layer 3 on the lowerelectrode 12, the drift layer 6 on the polycrystalline layer 3, and thesurface electrode 7 on the drift layer 6, are arranged, respectively, atthe lattice points of a matrix (lattice) formed by a group of thesurface electrodes 7 and a group of the lower electrodes 12 a. One ofthe pairs of the surface electrode 7 and the lower electrode 12 to beapplied with a certain voltage can be selected to allow electrons to beemitted from desired one of the electron source elements 10 a. Each ofthe lower electrodes 12 is formed in a strip shape, and provided with apad 28 on each of the longitudinal ends thereon. Each of the surfaceelectrodes 7 is also formed in a strip shape, and provided with a pad 27on the extended portion from each of the longitudinal ends thereof. Theelectron source elements 10 a are provided to pixels, respectively.

The operation of the electron source 10 concerning the eighth mode ofembodiment is approximately the same as that of the conventionalelectron source 10 in FIG. 22. Specifically, in this electron source 10,the surface electrodes 7 are disposed in a vacuum space, and a collectorelectrode (anode electrode) 21 is provided to a faceplate 30 disposed inoppose relation to the surface electrodes 7. A DC current Vps is appliedto allow selected one of the surface electrodes 7 to serve as a positivepole relative to the corresponding lower electrode 12, and a DC voltageVc is applied to allow the anode electrode to serve as a positive polerelative to the selected surface electrode 7. The resultingly generatedelectric field acts on the drift layer 6, and thus electrons injectedfrom the lower electrode 12 into the drift layer 6 is emitted throughthe surface electrode 7 after drifting in the drift layer 6.

As with the first mode of embodiment, the drift layer 6 has thestructure in FIG. 3. The electron source 10 concerning the eighth modeof embodiment emits electrons in the same mode as that in the first modeof embodiment. This electron source 10 allows electron beams emittedfrom the surface electrode 7 to uniformly direct in the normal directionof the surface electrode 7, and thus has no heed for providing a shadowmask or electron-focusing lens. Thus, the electron source 10 canfacilitate reducing the thickness of a display.

The electron source 10 concerning the eighth mode of embodiment can beproduced according to the production process in the fifth mode ofembodiment. For example, the drift layer 6 can be generally produced bythe following steps. A non-doped polycrystalline silicon layer is firstdeposited on the above entire principal surface of the insulativesubstrate 11. A portion of the polycrystalline silicon layercorresponding to the drift layer 6 is then anodized through the samenanocrystallization process as that in the fifth mode of embodiment toform a composite nanocrystal layer therein. Then, the compositenanocrystal layer is electrochemically oxidized through the sameoxidation process as that in the fifth mode of embodiment. In this way,the drift layer 6 is formed. While the nanocrystallization process andthe oxidation process in the eighth mode of embodiment are the same asthose in the fifth mode of embodiment, the nanocrystallization processand the oxidation process in the fifth or seventh mode of embodiment mayalso be used.

Further, as shown in FIG. 16, one or more current-feeding wirings 12 afor supplying current to the corresponding lower electrodes 12 locatedin the periphery of the polycrystalline silicon layer 3 as asemiconductor layer may be designed to have a width less than that ofcurrent-feeding wirings 12 a for supplying current to othercorresponding lower electrodes 12, so as to control the current densityof the principal surface of the semiconductor layer during theanodization and electrochemical oxidation treatments. In this case, ascompared to the conventional method, the variation in the currentdensity of the target region E can be reduced without modifying theshape of the cathode 33. Thus, the variation in the in-plane emissioncurrent Ie of the electron source 10 can be educed at a low cost.

[Ninth Mode of Embodiment]

A ninth mode of embodiment of the present invention will be describedbelow. An electrochemical oxidation method according to the ninth modeof embodiment will be described by taking the same electron source asthat in the first mode of embodiment for example, as one of electronicdevices to be formed through the anodization method and electrochemicaloxidation method. That is, the structure, function, advantages andoperation of electron emission of the electron source 10 concerning theninth mode of embodiment is the same as those of the electron source 10in the first mode of embodiment (see FIGS. 1 to 3). When the electronsource 10 concerning the ninth mode of embodiment is used as an electronsource of a display, a number of electron source elements 10 a may beformed on the side of one of the principal surfaces of a substrate 11 ina matrix arrangement by appropriately patterning the lower electrode 12,the surface electrode 7, drift layer 6 and others.

With reference to FIGS. 17A to 17D, a production process of thiselectron source 10 concerning the ninth mode of embodiment will bedescribed below. As with the first mode of embodiment, a lower electrode12 composed of a metal film is first formed on one of the principalsurfaces of an insulative substrate 11, and a non-doped polycrystallinesilicon layer 3 is formed on the entire principal surface of thesubstrate 11, to obtain a structure as shown in FIG. 17A.

After the formation of the polycrystalline silicon layer 3, thenanocrystallization process (anodization step) is performed to form acomposite nanocrystal layer 4 including a number of polycrystallinesilicon grains 51 (see FIG. 3) and a number of silicon microcrystals 63(see FIG. 3) which are mixed together. In this manner a structure asshown in FIG. 17B is obtained.

The nanocrystallization process is performed using an anodizationapparatus as shown in FIG. 24A to anodize the polycrystalline siliconlayer 3 as a semiconductor layer. After the completion of thenanocrystallization process, the oxidation process is performed toelectrochemically oxidize the composite nanocrystal layer 4. Throughthis process, the drift layer 6 composed of a composite nanocrystallayer with a structure as shown in FIG. 3 is formed to obtain astructure as shown in FIG. 17C. After the formation of the drift layer6, a surface electrode 7 composed of a gold thin film is formed on thedrift layer 6, for example, through a vapor deposition method. In thisway, an electron source 10 with a structure as shown in FIG. 17D isobtained.

The nanocrystallization process is the same as that in the first mode ofembodiment. In the oxidation process (oxidation step), anelectrochemical oxidation apparatus as shown in FIG. 18 is used toelectrochemically oxidized the composite nanocrystal layer 4 as asemiconductor layer (crystal layer). In the oxidation process, asolution comprising an organic solvent, e.g. ethylene glycol, and asolute of 0.04 mol/l potassium nitrate dissolved in the organic solventis used as a specific electrolytic solution B to be contained in aprocessing bath 31. An object 30 formed with the composite nanocrystallayer 4 is immersed into the electrolytic solution B. Then, a cathode 33is disposed in opposed relation to the composite nanocrystal layer 4within the electrolytic solution B. Then, a constant current (e.g. acurrent having a current density of 0.1 mA/cm²) is supplied from acurrent source between an anode (lower electrode 12) and the cathode 33to perform the oxidation process for electrochemically oxidizing thecomposite nanocrystal layer 4. Through this process, a drift layer 6including grains 51, silicon microcrystals 63 and silicon oxide films52, 64 (see FIG. 3) is formed.

During the oxidation treatment, the voltage between the anode and thecathode 33 is continuously detected by voltage detect means (not shown),and the oxidation treatment is terminated at the time when the voltagebetween the anode and the cathode 33 is increased by a desired voltagevalue from the voltage at the initiation of the oxidation treatment. Inthe period of supplying the current between the anode and cathode 33,the object 30 and the cathode are vibrated by the output of a vibrationgenerator 36. Thus, even if an electrochemical reaction forms airbubbles on the principal surface of the composite nanocrystal layer 4 ofthe object 30 and the surface of the cathode 33 in the current supplyperiod, the air bubbles will be immediately released therefrom, so as toprevent the air babbles formed on the principal surface of the compositenanocrystal layer 4 from masking the principal surface and suppressingthe electrochemical oxidation reaction or to prevent the air babblesfrom suppressing the reaction to be caused in theelectrochemical-oxidation target region. This technique makes itpossible to reduce the in-plane variation in the silicon oxide films 52,64 to be formed in the target region. In addition, this technique canprevent increase in the detected voltage from the voltage detect meansotherwise caused by the air bubbles formed on the cathode 33, andprevent deterioration in the withstand voltage performance of thesilicon oxide films 52, 64.

The vibration of the object 30 using the vibration generator 36 involvesthe risk of damage in a porous silicon layer. In view of thispossibility, instead of vibrating the object 30 using the vibrationgenerator 36, a vibrator (not shown) may be disposed in the electrolyticsolution B to vibrate the electrolytic solution B in the current supplyperiod so as to prevent air babbles generated through theelectrochemical oxidation reaction from remaining on the target regionand suppressing the electrochemical oxidation reaction, without damagein the porous silicon layer. Further, this technique can preventincrease in the detected voltage from the voltage detect means otherwisecaused by the air bubbles formed on the cathode 33, and thus preventdeterioration in the withstand voltage performance of the silicon oxidefilms 52, 64.

According to the ninth mode of embodiment, even if the electrochemicalreaction forms air bubbles generated on the principal surface of thecomposite nanocrystal layer 14 in the oxidation period, the air bubbleswill be immediately released therefrom so as to prevent the air babblesfrom masking the principal surface and suppressing the electrochemicaloxidation reaction. In addition, this method can provide reducedin-plane variation in the film thickness or film quality of the siliconoxide films 52, 64 to be formed in the electrochemical-oxidation targetregion. Thus, the in-plane variation in the withstand voltageperformance of the silicon oxide films 52, 64 can be reduced as comparedto the conventional method. Further, this method can prevent increase inthe detected voltage from the voltage detect means otherwise caused bythe air bubbles formed on the cathode 33, and thus prevent deteriorationin the withstand voltage performance of the silicon oxide films 52, 64.This allows the withstand voltage performance of the silicon oxide films52, 64 to be varied in each of lots.

The above technical concept can be applied to a nanocrystallizationprocess using an anodization method.

[Tenth Mode of Embodiment]

A tenth mode of embodiment of the present invention will be describedbelow. The electrochemical oxidation apparatus in FIG. 18 is used in theninth mode of embodiment. In the tenth mode of embodiments anelectrochemical oxidation apparatus as shown in FIG. 19 is used toelectrochemically oxidize a composite nanocrystal layer 4 of an object30. The construction and operation of an electron source 10 concerningthe tenth mode of embodiment is the same as those of the electron sourcein the ninth mode of embodiment, and their drawing and description willbe omitted. The production process of the electron source 10 concerningthe tenth mode of embodiment is fundamentally the same as that in theninth mode of embodiment, and its description will be omitted.

The electrochemical oxidation apparatus concerning the tenth mode ofembodiment includes a lift pump 39 for pumping up an electrolyticsolution B in a processing bath. Then, the pumped-up electrolytesolution B is jetted out from a jet flow from a nozzle (not shown)toward the principle surface of the cathode 33 and the semiconductorlayer (polycrystalline silicon layer 3, composite nanocrystal layer 4)of the object 30. In the tenth mode of embodiment, the nozzle is movedto jet the electrolytic solution B to the entire surface of the cathode33 and the entire principal surface of the semiconductor layer of theobject 30. As with the ninth mode of embodiment, even if anelectrochemical reaction forms air bubbles on the principal surface ofthe composite nanocrystal layer 4 of the object 30 in a current supplyperiod, the air bubbles will be immediately released therefrom, so as toprevent the air babbles formed on the principal surface of the compositenanocrystal layer 4 from masking the principal surface and suppressingthe electrochemical oxidation reaction. Thus, this method can providereduced in-plane variation in the film thickness or film quality of thesilicon oxide films 52, 64 to be formed in the electrochemical-oxidationtarget region, and the in-plane variation in the withstand voltageperformance of the silicon oxide films 52, 64 can be reduced as comparedto the conventional method.

Further, this method can prevent increase in the detected voltage fromthe voltage detect means otherwise caused by the air bubbles formed onthe cathode 33, and thus prevent deterioration in the withstand voltageperformance of the silicon oxide films 52, 64. This allows the withstandvoltage performance of the silicon oxide films 52, 64 to be varied ineach of lots. Furthermore, in the tenth mode of embodiment, theelectrolytic solution can be jetted out toward the principle surface ofthe semiconductor layer to release the air babbles from the principlesurface of the semiconductor layer. Thus, the air babbles formed on theprinciple surface of the semiconductor layer can be more reliablereleased therefrom.

The above technical concept can be applied to a nanocrystallizationprocess using an anodization method.

In either of the embodiments, the electrochemical oxidation apparatusmay be used as anodization apparatus only by changing an electrolyticsolution and incorporating some elements, such as light source, requiredfor anodization.

While the present invention has been described by reference to specificembodiments, various modifications and alterations will become apparentto those skilled in the art. Therefore, it is intended that the presentinvention is not limited to the illustrative embodiments herein, butonly by the appended claims and their equivalents.

INDUSTRIAL APPLICABILITY

As mentioned above, the electrochemical oxidation method according tothe present invention is useful, particularly, in a production processfor semiconductor devices such as field-emission-type electron sources,and suitable for use in an oxidation process in semiconductorproduction.

What is claimed is:
 1. A method of performing electrochemical oxidationto a semiconductor layer, wherein an electrode provided on an oppositeside of a principal surface to be electrochemically oxidized of saidsemiconductor layer is used as an anode, and a current is suppliedbetween said anode and a cathode while allowing said semiconductor layerand said cathode to be in contact with an electrolytic solution, tooxidize said semiconductor layer, said method comprising: supplying acurrent between said anode and said cathode to initiate said oxidation;and terminating said oxidation under such condition that a correctedvoltage value Vt determined by correcting a voltage V between said anodeand said cathode in accordance with a voltage inclement V0 based on apre-detected resistance of said electrolytic solution is equal to apredetermined upper voltage value V1.
 2. The method according to claim1, wherein said oxidation is terminated at a time point when thecorrected voltage value Vt has become equal to the upper voltage valueV1.
 3. The method according to claim 1, wherein said current is constantuntil the corrected voltage value Vt has become equal to the uppervoltage value V1, and is subsequently reduced down to a given valuewhile maintaining the corrected voltage value Vt at the upper voltagevalue V1, wherein said oxidation is terminated at a time point when saidcurrent has been reduced down to said given value, and during thereduction of said current, the corrected voltage value Vt iscontinuously determined by correcting the voltage V in accordance withthe voltage inclement V0.
 4. The method according to claim 1, furthercomprising detecting a resistance of said electrolytic solution using aresistance-measuring electrode before said current is supplied betweensaid anode and said cathode.
 5. The method according to claim 1, furthercomprising detecting a resistance of said electrolytic solution using aresistance-measuring region before said current is supplied between saidanode and said cathode, said resistance-measuring region being providedin the principal surface of said semiconductor layer in addition to agiven oxidization target region therein.
 6. The method according toclaim 1, further comprising detecting a resistance of said electrolyticsolution using a resistance-monitoring specimen before said current issupplied between said anode and said cathode, said resistance-monitoringspecimen having a shape identical to an object to be treated, formedwith said semiconductor layer.
 7. The method according to claim 1,wherein said electrolytic solution comprises an organic solvent and anelectrolyte dissolved in said organic solvent.
 8. The method accordingto claim 1, wherein a current density in the principal surface of saidsemiconductor layer is controlled in such a manner that the currentdensity in a periphery of an oxidation target region of saidsemiconductor layer is restrained in increasing to be greater than thatin the remaining oxidation target region.
 9. The method according toclaim 8, wherein the current density is controlled by forming saidcathode so as to have a shape which allows a distance between saidcathode and said semiconductor layer to be increased in a periphery ofsaid cathode.
 10. The method according to claim 8, wherein the currentdensity is controlled by forming said cathode so as to have a shapewhich allows a specific surface area per unit area in a periphery ofsaid cathode to be less than that in the remaining region of saidcathode.
 11. The method according to claim 8, wherein the currentdensity is controlled by providing a dummy region capable of reducingthe current density in the periphery of the oxidation target region ofsaid semiconductor layer.
 12. The method according to claim 8, in whicha plurality of said electrodes are arranged in parallel to one anotheron a surface of said semiconductor layer on the opposite side of theprincipal surface, each of said electrodes including a current-feedingwiring for feeding said current to said electrodes, wherein the currentdensity is controlled by forming the current-feeding wiringscorresponding to the periphery of said oxidation target region so as tohave an interval less than that of the current-feeding wiringscorresponding to the remaining portion of said oxidation target region.13. The method according to claim 1, wherein air bubbles formed on theprincipal surface of said semiconductor layer during the supply of saidcurrent, are released from the principal surface while supplying saidcurrent.
 14. The method according to claim 13, wherein the air bubblesare released by vibrating a substrate having said anode and saidsemiconductor layer.
 15. The method according to claim 13, wherein theair bubbles are released by giving vibration from a vibrator disposed insaid electrolytic solution, to said electrolytic solution.
 16. Themethod according to claim 13, wherein the air bubbles are released bydirecting a jet flow of said electrolytic solution toward the principalsurface of said semiconductor layer.